The Geekiest One

Igneous Rocks and Magma

1) Introduction

Igneous rocks are defined as those rocks which have crystallised
from a silicate melt (magma) either within the Earth or at the surface. If the magma cools at depth, slow cooling will occur and a coarse-grained igneous rock will result. If cooling is more rapid, a medium-grained rock is formed ( for example at shallow depths – the hypabyssal environment – sills, centres of dykes), and if the magma erupts at the surface, cooling is very quick so a fine-grained volcanic, rock is formed. With very rapid chilling, a volcanic glass can be formed (obsidian).

Magma contains dissolved gases that remain in solution under pressure. When the pressure is released this can lead to an eruption. This pressure release is called exsolution. Such a process can lead to a fragmental or frothy rock being erupted called a pyroclastic eruption. Magma erupting underwater (sea, lake) produces characteristic shapes known as pillow lavas. These have glassy chilled margins with interiors full of holes, where gases were exsolved but trapped within the rock. The holes are termed vesicles (the sites of former gas bubbles) and are also common in lava flows.

Magmas intruded into small fissures near the surface of the Earth form dykes if vertical and discordant to the bedding, and sills if near horizontal and concordant with the bedding. These dykes and sills may have chilled margins and coarser interiors, and often show cooling joints perpendicular to the cooling surfaces.

2) Classification of igneous rocks

Igneous rocks may be classified using grain-size.
This classification roughly corresponds to plutonic, hypabyssal and volcanic environments. The crystals of coarse-grained rocks can be seen easily with the naked eye; those of medium-grained rocks need a hand-lens; those of fine-grained rocks require a microscope to be resolved.

Some igneous rocks show evidence of having undergone two stages of cooling. The magma may pause en route to the final place of intrusion or extrusion, and cool slightly. Crystals will form and grow. Subsequently eruption or final intrusion takes place and the remainder of the magma will crystallise with a generally finer- grained texture. So we see large crystals (termed phenocrysts) in a finer-grained matrix (called the groundmass). This is known as porphyritic texture.

Igneous rocks can also be classified using mineralogy and chemistry. Textural terms (above) are also useful because the same magma (for example a magma of basaltic composition) can crystallise under different conditions to give very different looking rocks. The chemical composition of the rocks will be the same, and the mineralogy may or may not be similar depending on the physical history, but the rocks may look very different.

3) Chemical definitions

Chemical analysis of igneous rocks gives a classification according to their chemical compositions. A common classification is based on the amount of silica in the rock. Note that this is the chemical amount of silicon dioxide (SiO2), not the quantity of the mineral quartz.

4) Mineralogical definitions

Igneous rocks are composed of varying percentages of mafic (ferromagnesian) minerals such as olivine, pyroxene, amphibole, biotite mica, and felsic minerals such as plagioclase, alkali feldspar and quartz. Generally, mafic minerals tend to be dark in colour and felsic ones light in colour, although this is not always the case.

The percentage of mafic minerals in a rock is called the Colour Index (C.I.) A high colour index is associated with ultrabasic and basic rocks containing >50% mafic minerals. These rocks are sometimes referred to as melanocratic. Colour indices of 30 – 50% are referred to as mesocratic, and are associated with intermediate rocks, while a low C.I. of <30%, referred to as leucocratic, is associated with acid rocks. Do not be led astray by the black obsidian glass which is not a mafic mineral.

We define igneous rocks using a combination of the percentages of quartz, alkali feldspars, plagioclase feldspars and ferromagnesian (mafic) minerals. Grain size also plays a part in naming igneous rocks.

Ultramafic rocks; These contain nearly 100% ferromagnesian minerals.

Dunite is a rock which is rich in the mineral olivine. Pyroxenite is a rock rich in the mineral pyroxene. Peridotite is a rock consisting mainly of olivine and pyroxene. All of these are intrusive rocks and therefore have a coarse or medium grain size. Extrusive rocks of this composition and mineralogy are called komatiites. These are rare today, but more abundant in the early history of the Earth when the mantle was hotter.

Basic rocks:

Basic rocks contain approximately 50% plagioclase (composition typically of labradorite >An50) and 50% mafic minerals (pyroxene). Olivine may also be present, in which case we can preface the name of the rock with the mineral name, for example, olivine basalt. Coarse-grained rocks of this composition are called gabbros; medium-grained ones are dolerite, and fine-grained ones are basalts.

Intermediate rocks:

The diorites are characterised by felsic minerals such as plagioclase (usually andesine), and ferromagnesian minerals (mafics) which may include hornblende and biotite (rarely pyroxene). The ratios are generally such that plagioclase is more abundant than mafics. Quartz and alkali feldspars also may be present. In coarse-grained form they are termed diorite or quartz diorite, and when fine-grained, andesite or dacite.

Acidic rocks:

Quartz and alkali feldspar are abundant (>50% of the total). Plagioclase is andesine or oligoclase. Alkali feldspar includes albite, microcline, orthoclase. Mafic minerals are less abundant and are commonly biotite or hornblende. Muscovite (white mica) also occurs in some granites. Coarse-grained rocks of this composition are called granodiorite or granites and fine-grained varieties are rhyodacite or rhyolite. Other types occur depending on the cooling history.

The relationship between SiO2 and the relative proportions of different minerals is illustrated by a diagrammatic model. If a sample has 50% silica, then using the model it could be ascertained that it should contain 5% olivine, 70% pyroxene and 25% plagioclase feldspar. If it is coarse-grained it would be a gabbro and if fine, a basalt.

5) Origin of basaltic magmas

Magmas are formed by the melting of pre-existing rocks. This occurs in the upper mantle, perhaps where water has lowered the melting temperature. The upper mantle is composed of peridotite.

Peridotite is a complex solid, commonly consisting of 4 minerals (olivine, clinopyroxene, orthopyroxene and spinel or garnet). Each of these minerals melts at a different temperature and so peridotite does not completely melt at any single temperature.

As the temperature rises, some minerals melt and others remain solid. The melting process is aided by the presence of water or by the release of pressure. The parts that melt first (consisting of the minerals with the lowest melting points) rise to higher crustal levels to become rocks of different overall compositions.

Mantle peridotites have been subjected to melting experiments at appropriate pressures in the laboratory, and basaltic magmas are produced in such experiments. Therefore, low P and S wave velocities in the upper mantle are attributed to local basaltic melts. The process of melting a solid to form a melt of different composition is called partial melting.

Once basaltic magmas have been formed by partial melting of the mantle, they rise upward towards the surface of the Earth. This is due to their buoyancy arising from the lower density of the partial melt than that of its parent. During the process of rising magmas cool and crystals begin to form. If these crystals are removed from the melt (for example they fall to the floor of the magma chamber) or are otherwise prevented from continuously reacting with the magma (formation of zoned crystals), then the magma will change in composition. This is the process of fractional crystallisation and it leads to the formation of families of related igneous rocks (sometimes called a suite or a rock series). Rocks such as basalts, andesites, dacites and rhyolites may all be erupted from a volcano beneath which a magma chamber is undergoing fractional crystallisation.This process can be repeated until acidic rocks (rhyolites) are formed. However, most granitic magmas are probably formed by another process: partial melting of crustal rocks.